Kefka can be defeated my spawning Flood after building up a decent amount of Bravery, although you're better off using Tornado in small corridors. Kefka's Magic Pot Summon can mimic your bravery, so you should never build up too much of it as you'll risk turning the tables instantly. Try and dodge Kefka's Trine HP attack as much as you can while pounding him with bravery attacks to counter his other attacks.